In the first year of this project we initiated three sub-projects designed to provide a baseline understanding of how this system develops as well as to begin to identify at least some of the factors that mediate innervation within the cochlea. First, we began a descriptive study of the formation and differenation of spiral ganglion neurons using several antibody markers. Preliminary results indicate neuronal processes inter the developing cochlea prior to hair cell formation, but that the formation of specific contacts is dependent on the presence of developing hair cells. In a second series of experiments we used an genetic approach to label a small subset of developing spiral gangion neurons with yellow fluorescent protein. This approach will allow us to visualize single neurites as the establish connections with hair cells. In the future we will combine this technique with time lapse imagery to observe how specific connections are established Finally, as a third approach we have examined the effects of a population of mesenchymal cells that spiral ganglion neurites must pass through prior to entering the cochlea. Analysis of mice with a mutation in Pou3f4, a known deafness gene expressed exclusively in mesenchyme, indicates defects in neuronal pathfinding, indicating that mesenchymal cells play a key role in cochlear development.